The Montreal protocol for the protection of the ozone layer led to the abandoning of the use of chlorofluorocarbons (CFCs). Compounds less harmful for the ozone layer, such as hydrofluorocarbons (HFCs), therefore replaced the chlorofluorocarbons. These compounds, however, are relatively substantial contributors to the greenhouse effect. There is therefore a need for effective compounds combining a low ODP (ozone-depleting potential) with a low GWP (global warming potential). Hydrofluoroolefins (HFOs) have been identified as desirable alternatives, by virtue of their low ODP and GWP values.
The compound 2,3,3,3-tetrafluoropropene (HFO-1234yf) is of particular interest in this regard.
A number of documents relate to processes for preparing fluorinated olefins from pentachloropropane.
For example, document US 2009/0240090 describes the reaction of 1,1,1,2,3-pentachloropropane (HCC-240db) to give 2-chloro-3,3,3-trifluoropropene (HCFO-1233xf) in the absence of oxygen. The resulting HCFO-1233xf is converted to 2-chloro-1,1,1,2-tetrafluoropropane (HCFC-244bb).
Document WO 2009/015317 describes the reaction of a chlorinated compound such as 1,1,2,3-tetrachloropropene (HCO-1230xa), HCC-240db, or 2,3,3,3-tetrachloropropene (HCO-1230xf) with hydrogen fluoride (HF), in the gas phase. This process can be used to obtain HCFO-1233xf.
Document WO 2005/108334 describes the reaction of HCC-240db with HF to give 2-chloro-1,1,1,3-tetrafluoropropane (HCFC-244db), which is then dehydrochlorinated to give 1,3,3,3-tetrafluoropropene (HFO-1234ze).
Document WO 2010/123148 describes the fluorination of HCC-240db to HCFO-1233xf in the absence of catalyst.
Patent application PCT/IB2010/003028, filed by the applicant, describes the catalytic gas-phase fluorination of HCC-240db or of 1,1,2,2,3-pentachloropropane (HCC-240aa) to HFO-1234yf.
In the context of this process, however, it is necessary to separate the compounds obtained from the reaction step, namely the desired HFO-1234yf product, other fluorinated compounds formed during the reaction, hydrochloric acid (HCl), which is also formed during the reaction, the unreacted reactants (particularly HF), degradation products, or inert compounds.
In order to do this, it is possible to use an assembly of distillation columns that produces a stream of HFO-1234yf, an HCl stream, and a stream intended for recycling to the reaction. But the boiling points of HCl and of HFO-1234yf are relatively low (−85° C. for HCl at atmospheric pressure and −29.1° C. for HFO-1234yf at atmospheric pressure). Condensing the gases at the top of the distillation columns therefore requires a high operating pressure, so that the temperature needed at the top of the column is not too low, and therefore so that the process is compatible with the use of standard cold units.
Typically, the operating pressure required for distillative separation is greater than 5 bar, or even greater than 7 bar. The reactor must therefore be operated at an even higher pressure. This presents problems in so far as, in certain configurations, it is desirable to operate at a relatively low pressure in the reactor.
If the fluorination reaction is carried out at a lower pressure, the gases obtained from the reaction have to be compressed before distillation. That, however, implies an excessive compressor size, owing to the substantial excess of HF used in the fluorination reaction. This therefore represents a technology which is very complex to employ.
Consequently there is a genuine need to develop a method for producing HFO-1234yf wherein the fluorination reaction can be implemented at a moderate pressure.